Suppose we need to mock such service and verify that doSomething is called exactly once, and then no interaction happens with the service:

class SomeService {
    def doSomething(from: Int = 0, to: Int = 10): Unit = {
      //... implementation ...

    //... other methods ...

A common approach to do it:

val serviceMock = Mockito.mock(classOf[SomeService])

//run tested code which invokes this:

Mockito.verify(serviceMock).search(0, 10)

But here’s surprise: it fails with such error message:

Argument(s) are different! Wanted:, 10);
-> at .....
Actual invocation has different arguments:, 0);
-> at ....

Somehow, default params weren’t picked - instead doSomething was called with zeros. The mistery continues if we replace Mockito.verify line with this one:

Mockito.verify(serviceMock).search(Matchers.anyInt(), Matchers.anyInt())

Now error message tells us:

No interactions wanted here:
-> at ...
But found this interaction:
-> at ...
For your reference, here is the list of all invocations ([?] - means unverified).
1. [?]-> at ...
2. [?]-> at ...
3. -> at ...

We honestly call doSomething() one time and never call other methods of SomeService. Where from did other 2 calls appear then? Time to look at actual Java representation of default params. Go to the target folder of the project and find the folder with SomeService.class. Then run `javap SomeService’. You’ll find something like this in the output:

public void doSomething(int, int);
public int doSomething$default$1();
public int doSomething$default$2();

That pretty much explains our issues: when you omit parameters and expect to have default values, Scala compiler adds behind the scenes calls to the methods which return those parameters. That’s how additional 2 calls appear. And as the mock doesn’t have return values specified for those methods, it just returns zeros.